SBIR-STTR Award

The Low Gain Avalanche Detector LGAD), a new type of solid-state detector, has achieved a timing resolution of better than 20 psec, which has enabled the development of fast timing layers for the ATLAS and CMS detectors. The high speed of LGAD signals have also drawn the attention of the nuclear and photon science communities. In their current realization, however, the high fields needed to induce the LGAD gain are produced by a highly doped layer at or near the segmented implant layer. This leads to breakdown and requires the addition of an isolation structure the Junction Terminating Extension, or JTE). However, the JTE generates a dead region in the device, limiting the granularity of conventional LGADs to the 1x1 mm2 scale. On the other hand, the requirements of X-ray imaging, and of true four-dimensional tracking of beam collision products rather than the currently-planned time-stamp provided by a timing layer at the outside of the tracker) will require granularity at the 50x50 mm2 scale – over an order of magnitude reduction in the linear dimension of the LGAD pixel size relative to the sensors currently under development for the HL LHC. While there are several approaches to increasing LGAD granularity that are under exploration AC LGADs, Trenched LGADs, ILGADs), none of these have yet been shown to provide an adequate solution to the X-ray imaging and 4D tracking granularity challenge. Here, we propose to fabricate the first-ever prototype of a proprietary new approach to the production of high- granularity LGADs that permits conventional approaches to silicon diode pixilation to be employed, effectively removing the granularity limit suffered by the LGAD sensors under development for the ATLAS and CMS detectors at the LHC. Simulations performed with the Sentaurus Device simulation package suggest that the resulting sensor will satisfy essential goals of Topic 34b, achieving deep sub-millimeter granularity with GHz counting rate capability. In addition, Figure 1 shows the simulated gain variation for the current device baseline; a gain uniformity across the profile of the device of better than ±5% is expected. As for all solid-state sensors, the dynamic range will be limited only by space-charge saturation of the bias field. Furthermore, a capability unique to LGAD sensors is a dynamic range tunable by up to two orders of magnitude via the externally-applied bias voltage.

The development of Low Gain Avalanche Detectors (LGADs) over the past five years has opened up the possibility of ultra-precise timing measurements (better than ?30 psec) and high-frame rate sensing and imaging (in excess of 500 MHz) for X-rays and high-energy charged particles. However, due to the need to avoid electrical breakdown between neighboring pixel, conventional LGADs include inter-pixel isolation structures that limit the granularity to the 1x1 mm2 scale. Here, we propose the continue development of the Deep Junction LGAD (DJ- LGAD), a novel approach that will allow segmentation at the 10x10 µm2 scale while maintaining the precise timing resolution characteristic of LGADs. In the DJ-LGAD approach, a highly- doped junction is formed several microns below the surface of the sensor, isolating the high-field gain region from the surface electrode structure, and thus avoiding electrical breakdown between electrodes without the inclusion of inter-pixel termination structures. Two fabrication techniques to form the deep junction will be explored at the lead company (Cactus Materials): wafer-to- wafer bonding and epitaxial growth. During Phase-I work, a program of TCAD simulation was undertaken that allowed the conceptual DJ-LGAD idea to be developed into a fabricable planar (non-segmented) device with a stable buried junction. Fabrication parameters, including implantation dose and incidence angle, junction termination geometry (including guard-ring structure), annealing parameters, and junction width and depth, were established by the simulation studies. Preliminary fabrication results suggest that both wafer bonding and epitaxy are viable approach to fabricate the devices. Wafer bonding technique would be a transformational technology as it can enable detectors with dissimilar materials (III-V) along with silicon materials. I-V measurement from Phase I results suggest interface between wafers are electrically conductive. Further study is in progress to fabricate a fully planar DJ-LGAD device by the end of Phase I. The Phase-II project will build on this, refining the wafer-to-wafer bonding approach and developing the epitaxial method. TCAD simulation studies will be used to extend the planar design of the Phase-I prototype to a fully granular design, which will then be fabricated and characterized with X-ray and proton beams. Commercialization strategies will also be developed. Applications within the realm of pure science include 4D and next-generation silicon strip tracking for further exploration of the fundamental building blocks of the universe and the structure and behavior of nuclear matter. Applications within the realm of applied science and technology include medical tomography and imaging, optoelectronic communication, photon and materials science (X-ray imaging and diffraction) and accelerator development.